Well the van you are looking at that you stated was a 2000(3rd gen) is probably a 2001(4th gen). 2001's won't just drop into 3rd gen. There are modifications that need to be done. Any 3.8 from a 1996-2000 will drop into your 1999 without a hitch, but any other 3.8 (2001+) will need modifications. UNLESS the 2000 you're talking about is actually a 2000 and the shop is mistaking it for a 2001+. Try to find out the year again and let us know.
2001 Infiniti i30 3.0 DOHC V6: VQ30DE 177,000 miles; 1999 Nissan Altima GLE KA24 168,000 Miles
1997 Dodge Ram 1500 SLT Laramie 5.9 46RE, 182,000 Miles
Prev. 97 T&C LXi 184k 3.8, 94 T&C 3.8 156k Miles, 93 GV LE 3.3 123k